Unions have a lot of ground to make up. In 1983, the first year for which comprehensive national data are available, 20% of American wage and salary workers were union members. That figure has fallen nearly every year since, hitting an all-time low of 11.3% last year. There were 14.4 million union members in the U.S. last year, 3.4 million fewer than in 1983, even as the total workforce has grown by 3.9 million. (Another 1.6 million workers are covered by union contracts but aren’t union members; in all, 12.5% of workers were represented by unions in 2012.)

Union membership has been buffeted by a range of forces. The manufacturing sector, long a redoubt of organized labor, has been in a prolonged decline, at least in terms of employment. And within manufacturing, union membership has fallen below 10% from nearly 15% a decade ago, due in part to a shift of factory activity to less union-friendly “right-to-work” states. (16.6% of workers in Michigan are unionized, for example, compared to 3.3% in South Carolina.)